A commercially available conventional lithium battery contains an organic liquid electrolyte in which an inflammable organic solvent is used, and therefore it is required to have a safety device to inhibit temperature increase at the time of short circuit or an improvement in structure and material for preventing short circuit. Meanwhile, an all solid battery in which a liquid electrolyte is changed to a solid electrolyte does not use any inflammable organic solvent in the battery, and therefore it is believed that a safety device can be simplified, and thus it is excellent in terms of production cost or productivity. In addition, an electronic circuit can be integrated by preparing a battery as a thin film. Moreover, because an inorganic solid electrolyte has ion selectivity, when an inorganic solid electrolyte is used, reliability of a battery including cycle lifetime, storage lifetime, or the like can be enhanced.
An all solid battery has excellent characteristics as described above. However, in general, the power density is lower than that of the liquid electrolyte systems, and this drawback must be overcome if the battery is to be used as general-purpose batteries, in particular.
In order to increase the output of the all solid lithium secondary battery, a solid electrolyte having high ion conductivity must be used. Solid electrolytes exhibiting an ion conductivity of 10−3 S/cm or more at room temperature include oxides such as LiTi2(PO4)3 having a NASICON structure, its analogue compounds, and (Li,La)TiO3 having a perovskite structure; and lithium nitride. However, the oxides above are compounds containing titanium which is susceptible to electrochemical reduction. Thus, when these oxides are used as the electrolyte of the all solid lithium battery, electronic conduction is generated according to the reduction of titanium and the compound can no longer function as an electrolyte. Meanwhile, lithium nitride has a decomposition voltage as low as 0.45 V and cannot be used as an electrolyte for a high-voltage lithium battery. In contrast to these solid electrolytes having low electrochemical stability, sulfide based solid electrolytes have both high ion conductivity and electrochemical stability.
However, according to the present inventors' review on electrochemical stability of the sulfide based solid electrolytes, it has been found that although the sulfide based solid electrolyte does not undergo continuous decomposition reaction upon application of a high potential, a high-resistance layer is formed at the interface at which the electrolyte contacts the cathode active material that shows electrode reaction at a high potential. It has also been found that this high resistance layer is the cause of incapacity to generate high output current. To solve the problems, by having a buffer layer of a material which functions as an oxide based solid electrolyte, such as Li4Ti5O12 or LiNbO3, between a sulfide solid electrolyte and a transition metal oxide such as LiCoO2 as a cathode active material, the inventors have succeeded in enhancing output performance of an all solid lithium battery so that it is comparable to a commercially available lithium ion battery (Patent Literature 1).
When an electrode active material contacts an electrolyte, migration of movable ions occurs in the contact interface due to a difference in electrochemical potential of the movable ions of these materials. Although the electrochemical potential of a lithium ion in a sulfide based solid electrolyte has not been determined so far, it is 2.5 V or less versus a lithium electrode, which is the oxidation potential of the sulfide ions. When such a solid electrolyte is brought into contact with a cathode active material of 4 V or more versus a lithium electrode, migration of lithium ions from the sulfide solid electrolyte to the cathode active material occurs, and as a result, equilibrium at interface is reached. In this case, since the difference in electrochemical potential of lithium ion is large between them, degree of mass migration of lithium ions is big. Further, since most of the cathode active materials have electron conduction, concentration gradient of movable ions is not likely to occur inside of them. Accordingly, a space charge layer depleted with lithium ions is significantly developed at the sulfide solid electrolyte side of the interface, and thus equilibrium is reached. Since very few lithium ions are present as a charge carrier in the space charge layer, it results in high resistance in an all solid lithium battery and impairs output performances.
In other words, the reason of forming a high resistance layer at an interface between a cathode active material and a sulfide solid electrolyte is, since the nearest ions are different between the cathode active material which is an oxide and the solid electrolyte which is a sulfide, the electrochemical potential of lithium ion is significantly different and the cathode active material has electron conduction. Thus, to prevent formation of such high resistance layer, the cathode active material and the sulfide solid electrolyte need to be brought into contact with each other at an interface at which those two conditions are not simultaneously satisfied.
For such reasons, according to Patent Literature 1, an oxide based solid electrolyte layer is interposed between LiCoO2 and a sulfide solid electrolyte. When an oxide based solid electrolyte is interposed between LiCoO2 and a sulfide solid electrolyte, two interfaces, i.e., an interface between LiCoO2 and an oxide based solid electrolyte and an interface between an oxide based solid electrolyte and a sulfide solid electrolyte, are generated.
The former is an interface at which oxides are in contact with each other and there is no big difference in electrochemical potential of lithium ions, which is a driving force for forming a space charge layer. Although the latter is an interface at which oxide and sulfide are in contact with each other, as being an interface between electron insulators, the space charge layer is not well developed into Schottky type, and thus, by using the oxide based solid electrolyte layer as a buffer layer for the interface, LiCoO2 and sulfide solid electrolytes can be brought into contact with each other under a state in which development of a space charge layer functioning as a high resistance component is inhibited, and as a result, output performances of an all solid lithium battery using a sulfide solid electrolyte can be improved greatly.
To have a large area of an interface between an electrode active material and a solid electrolyte at which an electrochemical reaction occurs, electrodes of an all solid lithium battery are generally a mixture of electrode active material powder and solid electrolyte powder. In this regard, in Patent Literature 1, a structure in which an oxide based solid electrolyte layer is interposed between an electrode active material and a sulfide solid electrolyte, which is obtained by forming a thin film of an oxide based solid electrolyte layer on surface of electrode active material powder by spray method and mixing it with a sulfide solid electrolyte powder, is disclosed.
According to the process, for forming a buffer layer of oxide based solid electrolyte by a spraying method, a method of forming an oxide based solid electrolyte layer including that alkoxide is used as a precursor for producing oxide based solid electrolyte, its alcohol solution is sprayed onto active material powder, and then the alkoxide is thermally decomposed by heating is employed. However, when such method is employed, although output performances of an all solid lithium battery are greatly improved, the problems to be solved as follows are yielded.
First, a process of forming a buffer layer by a spraying method is a batch type process with poor large scale productivity and also the alkoxide as a precursor for a buffer layer is prone to hydrolysis, difficult to handle, and expensive. Further, thickness of a buffer layer described as preferable in Patent Literature 1 is 100 nm or less, and especially, for obtaining an all solid lithium battery having high output performances, the thickness is very thin like 10 nm or so. For uniform formation of such thin layer on surface of an electrode active material particle, it is necessary to follow strictly the various conditions for forming a buffer layer.
Further, the buffer layer is formed by thermal decomposition of alkoxides, but during a heating treatment for performing thermal decomposition, inter-diffusion occurs at certain level between an electrode active material and a buffer layer. However, since the thickness of the buffer layer is desirably very thin, the inter-diffusion may yield loss of the buffer layer (K. Takada, N. Ohta, L. Zhang, K. Fukuda, I. Sakaguchi, R. Ma, M. Osada, and T. Sasaki, Solid State Ionics, 179, 1333-1337 (2008)), and thus it is necessary to perform very precisely the temperature control or the like for thermal decomposition of alkoxides.